Method and apparatus for displaying process endpoint signal based on emission concentration within a processing chamber

ABSTRACT

A method displays a signal which represents the variation of light intensity in a processing chamber over time. The light intensity within the process chamber is detected to produce a voltage signal with a voltage amplitude which varies based on the light intensity within the process chamber. The voltage amplitude of the voltage signal is digitally sampled at a sampling interval to obtain sample values. Running averages of a preselected number of sample values are calculated and displayed on a graph. For each newly displayed running average, a rectangular box may be superimposed over the graph of the running averages in order to graphically display changes in slope. A computer is programmed to look for an anticipated sequence of slopes.

This is a continuation-in-part of copending application U.S. Ser. No.07/799,693 filed Nov. 25, 1991, now abandoned, which is a continuationof U.S. Ser. No. 07/464,836 filed Jan. 16, 1990, now U.S. Pat. No.5,097,430 issued Mar. 17, 1992.

BACKGROUND

The present invention is directed to real time and off line signalprocessing of a signal indicating plasma concentration within aprocessing chamber.

In a processing chamber, for example an etch chamber used for performinga reactive ion etch, the concentration of plasma within the processingchamber is monitored to determine the end point of the process. Sincethe light intensity from the flow of the plasma is proportional to theconcentration of plasma within the processing chamber, the lightintensity within the processing chamber is monitored using a photomultiplier tube (PMT). The PMT generates an electrical signal with avoltage which is proportional to the light intensity. This signal isamplified and forwarded to a computer. The computer uses the signal tolook for certain parameters in order to intervene in the process, e.g.,to determine the end point of the process, when processing is to bestopped.

When a rotating magnetic field is used in the processing chamber, thiscauses plasma in the processing chamber to circulate. The circulation ofplasma causes significant variations in the light intensity measured bythe PMT. In order to accurately determine the end point of the process,the computer needs to filter the signal from the PMT to remove the noisecaused by effects of the circulation. It is desirable to generate adisplay to a user which represents the method by which the computerdetermines the end point of the process.

SUMMARY OF THE INVENTION

Light intensity within the process chamber is detected to produce avoltage signal with a voltage amplitude which varies based on the lightintensity within the process chamber. The voltage amplitude of thevoltage signal is digitally sampled at a sampling interval to obtainsample values. Running averages of a preselected number of sample valuesare calculated.

For each newly displayed running average, a rectangular box having aheight H and a width W may be superimposed over a graph of the runningaverages in order to graphically display changes in slope. When a newlydisplayed running average exits to the right of the rectangular box, thegraph is considered to have a zero slope.

When a newly displayed running average is above the rectangular box, thegraph is considered to have a positive slope. When a newly displayedrunning average is below the rectangular box, the graph is considered tohave a negative slope.

The computer is programmed to look for an anticipated sequence ofslopes. For example, the computer may look for positive slope, followedby a zero slope. The computer knows it has found a slope from theanticipated sequence of slopes when the graph for a preprogrammed numberof running averages is seen to have that slope. For example, if thecomputer is looking for a negative slope and the preprogrammed number istwo, when two successive running averages are below their respectiverectangular boxes, then the computer detects that the computer hasdetected the negative slope. The computer highlights the rectangular boxfor the running average at which the slope was detected. Thehighlighting may be done, for example, by changing the color of the boxon the display. The highlighted rectangular boxes are not erased. When anew rectangular box is displayed, prior rectangular boxes which have notbeen highlighted are erased.

Once the computer has detected a slope from the sequence of anticipatedslopes, it begins to look for the next slope in the sequence ofanticipated slopes.

For the first few running averages, the rectangular box is placed sothat the first running average bisects a left side of the rectangularbox. When a change of slope is detected, a new rectangular box is placedat the point of the exit from the prior rectangular box so that thepoint of exit from the old rectangular box bisects the left side of thenew rectangular box.

In accordance with a preferred embodiment of the present invention, amethod is presented for displaying a filtered signal which filters outthe effects of a magnetic field which causes plasma within theprocessing chamber to rotate. The magnetic field is generated by asignal with a period T. The period T is set equal to the preselectednumber of sample values times the sampling interval times an integer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a system used to detect light intensitywithin an etch chamber in accordance with the preferred embodiment ofthe present invention.

FIG. 2 is a block diagram of a system which provides a signal to acomputer based on the light intensity within the etch chamber detectedby the system shown in FIG. 1.

FIG. 3 is a block diagram of coils which create a magnetic field withinthe etch chamber shown in FIG. 1.

FIG. 4 is a graph which compares the signal provided to the computer bythe system shown in FIG. 2 before and after filtering by the computer inaccordance with the preferred embodiment of the present invention.

FIG. 5 shows how variation in the number of samples used to calculaterunning averages affects resolution of a displayed signal representinglight intensity with the etch chamber in accordance with the preferredembodiment of the present invention.

FIG. 6 through FIG. 23 illustrate the box method of measuring the slopeof a displayed signal by giving an example of the use of the box methodin accordance with the preferred embodiment of the present invention.

FIG. 24 gives a second example of the use of the box method inaccordance with the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, wafers to be processed are placed upon a wafer table 12within an etch chamber 10. Light generated by plasma within an innerregion 11 of the etch chamber 10 is carried through an optical fiber 13to an entrance slit 18 of an optical chamber 19. Light from the entranceslit 18 is reflected by a concave holographic grating 14 to an exit slit15. The dimensions of entrance slit 18 and exit slit 15 and/or the angleof the concave holographic grating 14 may be adjusted. The holographicgrating 14 serves to separate the wavelengths of light from opticalfiber 13 through the entrance slit 18 in order to monitor theconcentration of a single element within the inner region 11 of the etchchamber 10. A PMT 17 receives light from the exit slit 15 and convertsthe light intensity to an electrical signal.

As shown in FIG. 2, the electrical signal from PMT 17 is amplified by apre-amplifier 24. A voltage-to-frequency converter 25 generates a signalwith a frequency which varies with the voltage of the signal that thevoltage-to-frequency converter 25 receives from the pre-amplifier 24.The signal from the voltage-to-frequency converter 25 is fed to acounter 26. A computer 21 regularly reads the value of the counter 26,thus receiving a digital value representing the intensity of the lightsignal received by PMT 17. The counter 26 may be, for example, a 16 bitcounter read by a computer 21 every twenty milliseconds.

The anode potential of PMT 17, and thus the gain of the system, iscontrolled by the voltage of a signal generated by a power supply 23.The voltage of the signal generated by power supply 23 is controlled bythe computer 21 through a digital-to-analog converter 22.

FIG. 3 shows a processing chamber having a rotatable magnetic fieldaround region 11 of the chamber 10. Two pairs of coils, a coil 31, acoil 32, a coil 33 and a coil 34, are used to generate a magnetic fieldinside the chamber. A first signal having the form "A sin (2/T)t" isgenerated by a signal generator 35 and a second signal having the form"A cos (2/T)t" is generated by a signal generator 36, where "T"represents the period of the first signal and the second signal and "t"represents time. The first signal, circulating through coil 31 and coil33, and the second signal, circulating through coil 32 and 34, causeplasma within the inner region 11 of the etch chamber 10 to rotate in adirection 27 as shown.

FIG. 4 shows an unfiltered signal 43 received by the computer 21. Theunfiltered signal 43 is on a graph which records intensity along an axis42 and the passage of time along an axis 41. The periodic variation inthe signal 43 results from the rotation of the plasma within the etchchamber 10. The computer 21 can filter the signal 43 to produce afiltered signal 44 which represents the concentration of the plasmawithin the etch chamber 10.

Filtering of the signal 43 is done as follows: the computer 21 secures asample with eighteen bit resolution every one/tenth second. In thepreferred embodiment, the computer 21 samples and clears a 16-bitcounter 26 every 25 milliseconds. The computer 21 reads the counter 26four times and sums the results to obtain every one-tenth second asample value "S_(x) " with eighteen bit resolution. The computer 21calculates a running average "R_(y) " of a preselected number "N" ofsample values (S_(x)) where y is an integer representing the currentrunning average being calculated and, for each R_(y), x is an integervarying from y to y+(N-1). Specifically, ##EQU1##

The filtering algorithm works best when the period T of the first andsecond signal, as defined above, is an integer multiple of N times thesampling interval (I). That is,

    T=k* (N*I), where k=1, 2, 3, . . .

FIG. 5 shows how filtering becomes less than optimal when the abovealgorithm is varied. A filtered signal 52 on the graph 51 is calculatedby the computer 21 using the above optimal filtering algorithm and avalue for k of 1.0. A filtered signal 54 on the graph 53 is calculatedby the computer 21 setting k to the value of 1.05. A filtered signal 56on the graph 55 is calculated by the computer 21 setting k to the valueof 0.95. It is apparent that the filtering algorithm is optimized when kis an integer.

In the preferred embodiment of the present invention, when amagnetically enhanced processing chamber is employed, a graph of thefiltered signal is displayed by the computer 21. This may be done inreal time, as the data is being generated, or the data may be stored ina file and retrieved for use at a later time by the computer 21. Whenthe processing chamber does not have a rotating magnetic field, anunfiltered signal may be used.

The computer 21 utilizes the slope of the generated signal to makeprocessing decisions. For example, when the slope of the signal is aparticular value, the computer 21 may determine that an end point of aprocess within the etch chamber 10 has been reached. For the ease of auser, a box may be superimposed over the display of the signal. Thisgraphically shows a user how and when the computer 21 detects criticalpoints, for example an end point, of a process. FIGS. 6 through 23illustrate this.

In FIG. 6, a box 91 of width W and height H is shown superimposed over adisplay of a filtered signal 90. Width W is measured in seconds andheight H is measured in light intensity. Points within the signal 90show the value of a running average of light intensity as the runningaverage varies over time. Running averages R_(i) and R₂ are shown. Inreal time systems, a running average may be displayed, for example,every one-tenth second. In FIG. 6, however, a relatively large distanceexists between R₁ and R₂ in order to facilitate discussion of theoperation of the displayed boxes. In real time systems, R₁ and R₂ wouldbe spaced much closer together and resolution of the signal 90 would befar greater.

In a real time system, running averages (R_(y)) represent the calculatedrunning average of light intensity as defined above. In a real timeprocess these running averages are displayed sequentially by thecomputer 21.

Upon display of R₁, box 91 is displayed by the computer 21. Box 91 isplaced so that the center of the left side of the box 91 is at R₁. Box91 remains displayed until a signal 90 exits the box 91. If the signal90 exits on the bottom of box 91, the signal 90, at the point of exit,is considered to have a negative slope. If the signal 90 exits on thetop side of box 91, a signal 90 at the point of exit is considered tohave a positive slope. As long as (R₁ -R₂)<H/2, R₂ will remain in box91. For the purposes of the present invention, when R₂ is in box 91, thesignal at R₂ is considered to have zero slope. If R₁ <R₂ -H/2, then R₂will be above box 91 and the slope of the signal 90 at R₂ will beconsidered to be positive. If R₁ >R₂ +H/2, then R₂ will be below box 91and the slope of the signal 90 at R₂ will be considered to be negative.

Starting with running average R₂ and continuing for each running averagethereafter, the computer 21 is programmed to wait for either a zero, apositive or a negative slope. For example, since R₂ is within box 91,and is not at the right side of box 91, the signal 90 is not exiting box91 yet, so there is no slope. If the computer 21 were programmed to waitfor a zero slope, the computer 21 would have to wait until the signal 90came to the edge of box 91. Box 91 would then be highlighted and leftunerased. The computer 21 would then start a new box at the point ofexit of the signal 90 and look for the next programmed condition, ifany. When the next running average was displayed, a new box would havebeen displayed so that the center of the left side of the new box wouldhave been at the exit point of the box 91.

The operation of the invention will be described in detail in theexample which follows, as shown in FIGS. 6-23. In the present example,however, the computer 21 is programmed to look first for a negativeslope. After the negative slope has been found, then the computer 21 isprogrammed to look for a zero slope. Highlighted boxes will mark theplace where first the negative slope and then the zero slope is found.When the zero slope is found after the negative slope, this signifiesthe end point of the process.

In FIG. 7, a running average R₃ is displayed. When R₃ is displayed, box91 is not erased since R₃ is not to the right of the right side of box91. As long as R₁ -R₃ <H/2, then R₃ will not exit the top or the bottomof box 91. As long as the number of running averages calculated sinceentering box 91 (Y₉₁ times the sampling interval) is less than box widthW, the signal 90 will not exit the right hand side of box 91. If R₁ <R₃-H/2, then R₃ will be above box 91 and the slope of the signal 90 willbe considered to be positive. If R₁ >R₃ +H/2, then R₃ will be below box91 and the slope of the signal 90 will be considered to be negative.

Since the slope at R is not negative (i.e., as long as it is positive orzero), the computer 21 will continue the process as described herein.However, had R₃ been below box 91, then box 91 would have beenhighlighted and remain unerased. When the next running average wouldhave been displayed, a new box would have been displayed so that thecenter of the left side of the new box would have been at the point (R₃)at which the signal 90 exited box 91.

In FIG. 8, running averages R₄ and R₅ re displayed. When R₄ isdisplayed, box 91 is not erased since R₄ is not to the right of theright side of box 91. As long as R₁ -R₄ <H/2 and Y₉₁ *I<W, then R₄ willremain in box 91. If R₁ >R₄ +H/2 then R₄ will be below box 91 and theslope of the signal 90 will be considered to be negative. Similarly,when R₅ is displayed, box 91 is not erased since R₅ is not to the rightof the right side of box 91. As long as R₁ -R₅ <H/2, then R₅ will remainin box 91. If R₁ <R₅ -H/2 then R₅ will be above box 91 and the slope ofthe signal 90 will be considered to be positive. If R₁ >R₅ +H/2, then R₅will be below box 91 and the slope of the signal 90 will be consideredto be negative.

In FIG. 9, running averages R₆ and R₇ are displayed. When R₆ isdisplayed, box 91 is not erased since R₆ is not to the right of theright side of box 91. As long as R₁ -R₆ <H/2 and Y₉₁ *I<W, then R₆ willremain in box 91. If R₁ <R₆ -H/2 then R₆ will be above box 91 and theslope of the signal 90 will be considered to be positive. If R₁ >R₆+H/2, then R₆ will be below box 91 and the slope of the signal 90 willbe considered to be negative. However, when R₇ is displayed, box 91 isno longer able to contain all the displayed running averages (Y₉₁ *I<W).Box 91 is therefore erased and a new box 92 is displayed so that thecenter of the left side of box 91 is at R₂. As long as R₂ -R₇ <H/2 andY₉₂ *I<W, then R₇ will remain in box 92. If R₂ >R₇ -H/2, then R₇ will bebelow box 92 and the slope of signal 91 will be considered to benegative. However, the computer 21 is still waiting for a negativeslope.

In FIG. 10 a running average R₈ is displayed. When R₈ is displayed, Y₉₂*I<W, so box 92 is erased and a new box 93 is displayed so that thecenter of the left side of box 93 is at R₃. As long as R₃ -R₈ <H/2 andY₉₂ *I<W, then R₈ will remain in box 93. If R₃ <R₈ -H/2, then R₈ will beabove box 93 and the slope of the signal 93 will be considered to bepositive. If R₃ >R₈ +H/2 then R₈ will be below box 93 and the slope ofthe signal 90 will be considered to be negative.

In FIG. 11 a running average R₉ is displayed. When R₉ is displayed, Y₉₂*I>W, so box 93 is erased and a new box 94 is displayed so that thecenter of the left side of box 94 is at R₄. As long as R₄ -R₉ <H/2 andY₉₄ *I<W, then R₉ will remain in box 94. If R₄ <R₉ -H/2 then R₉ will beabove box 94 and the slope of signal 90 will be considered to bepositive. If R₄ >R₉ +H/2 then R₉ will be below box 94 and the slope ofthe signal 90 will be considered to be negative.

In FIG. 12 running averages R₁₀ and R₁₁ are displayed. When R₁₀ isinitially displayed, box 94 is erased and a new box is displayed so thatthe center of the left side of the new box is at R₅. When R₁₁ isdisplayed, the new box is erased and another new box 96 is displayed sothat the center of the left side of box 96 is at R₆. The computer 21 isstill waiting for a negative slope.

In FIG. 13 a running average R₁₂ is displayed. When R₁₂ is displayed,box 96 is erased and a new box 97 is displayed so that the center of theleft side of box 97 is at R₇. Since R₇ R₁₂ +H/2, R₁₂ is below box 97 andthe slope of the signal 90 is considered to be negative. This satisfiesthe condition waited for by the computer 21 described above. At thispoint, box 97 is highlighted, for example by changing the color of box97. This indicates to those watching the display the point at which theslope of signal 90 becomes negative. The computer 21 is now waiting fora zero slope in accordance with the preprogrammed condition looked for.

In the present example, one box which has a negative slope is sufficientto fulfill the condition. The computer 21 may also be programmed to waitfor two or more consecutive negative slopes before indicating that thecondition has been fulfilled. In this case, each of the consecutiveboxes in which the condition is fulfilled may be highlighted and leftunerased.

Since the waited for condition was fulfilled by running average R₁₂, avertical line 89 is drawn through R₁₂, indicating a change in slope.

In FIG. 14, a running average R₁₃ is displayed. When R₁₃ is displayed, anew box 102 is displayed so that the center of the left side of box 102is at R₁₂. Since box 97 fulfilled the waited for condition of a negativeslope, the left side of the next box (box 102) is at the point at whichsignal 90 exited box 97, i.e., R₁₂. Box 97 is not erased because itindicates the point at which the slope of signal 90 became negative,satisfying the condition first waited for by the computer 21. Since R₁₂R₁₃ +H/2, R₁₃ is below box 98 and the slope of the signal 90 remainsnegative. The condition waited for now is a zero slope; therefore, box102 is not highlighted.

In FIG. 15, a running average R₁₄ is displayed. When R₁₄ is displayed,box 102 remains since the condition of a zero slope has not been met andY₁₀₂ *I<W. Since R₁₂ >R₁₄ +H/2, R₁₄ is below box 99 and the slope of thesignal 90 remains negative. Box 102, therefore, is not highlighted.

In FIG. 16 a running average R₁₅ is displayed. Box 102 remainsdisplayed. Since R₁₂ >R₁₅ +H/2, R₁₅ is below box 102 and the slope ofthe signal 90 remains negative. Box 102, therefore is not highlighted.

In FIG. 17 a running average R₁₆ is displayed. When R₁₆ is displayed,box 102 remains displayed. Since R₁₂ >R₁₆ +H/2, R₁₆ is below box 102 andthe slope of the signal 90 remains negative. Box 102, therefore, is nothighlighted.

In FIG. 18, running averages R₁₇ and R₁₈ are displayed. When R₁₇ isdisplayed, box 102 remains displayed. When R₁₈ is displayed, Y₁₀₂ *I>W,therefore box 102 is erased and a new box 103 is displayed so that thecenter of the left side of box 103 is at R₁₃. Since R₁₃ >R₁₈ +H/2, R₁₈is below box 103 and the slope of the signal 90 remains negative. Box103, therefore, is not highlighted.

In FIG. 19, running averages R₁₉ and R₂₀ are displayed. When R₁₉ isdisplayed, box 103 is erased and a new box is displayed so that thecenter of the left side of the new box is at R₁₄. When R₂₀ is displayed,the new box is erased and another new box 105 is displayed so that thecenter of the left side of box 105 is at R₁₅. Since R₁₅ >R₂₀ +H/2, R₂₀is below box 105 and the slope of the signal 90 remains negative. Box105, therefore, is not highlighted.

In FIG. 20, a running average R₂₁ is displayed. When R₂₁ is displayed,box 105 is erased and a new box 106 is displayed so that the center ofthe left side of box 106 is at R₁₆. Since R₁₆ >R₂₁ +H/2, R₂₁ is belowbox 106 and the slope of the signal 90 remains negative. Box 106,therefore, is not highlighted.

In FIG. 21, a running average R₂₂ is displayed. When R₂₂ is displayed, anew box 107 is displayed so that the center of the left side of box 107is at R₁₇. Since R₁₇ -R₂₂ <H/2, R₂₂ is within box 107 and the slope atR₂₂ is zero. This is the condition the computer 21 has been waiting for.Box 107, therefore, is highlighted. Box 106 is erased as it is the lastbox with a negative slope. Since box 107 is the first box to satisfy thesecond condition sought by the computer 21, it will not be erased.

The computer 21 could be programmed to leave unerased a box which was apredetermined number of boxes after box 107 along with or instead ofhighlighting box 107. In the present example, only the box in which acondition is first satisfied remains unerased.

A vertical line 88 which intersects R₂₂ is added to indicate that theslope became zero at R₂₂.

In FIG. 22, running averages R₂₃ and R₂₄ are displayed. When R₂₃ isdisplayed, a new box 112 is displayed so that the center of the leftside of box 112 is at R₂₂. This is because R₂₂ was the location wheresignal 90 exited a box and fulfilled a waited for condition. R₂₂ -R₂₄<H/2; therefore, R₂₄ is within box 112. No condition is waited for andtherefore box 112 is not highlighted.

In FIG. 23, running averages R₂₅, R₂₆ and R₂₇ are displayed. Box 112 isdisplayed and not highlighted.

In order to change the location where the box method records a negativeor positive slope, the value of width W and height H may be changed.When the above box method is used to determine critical stages of aprocess, the computer 21 may store the data received from the counter 26in a file. This data may then be utilized by an operator to make manysimulated runs of the process. The width W and height H of the box canbe adjusted until the box will become highlighted at precisely the pointdesired by the operator. The width W and height H determined by theoperator can be used by the computer 21 to control real time processes.

The box method can be used to detect several critical stages in a singleprocess requiring a controller of the process to take action. Thecomputer 21 can be programmed to look for any combination of a positiveslope, a negative slope, or a zero slope. For example, in FIG. 24 asignal 120 is shown. For a process such as that represented by FIG. 24,it may be desirable to detect five changes in slope. The computer 21would be programmed to look first for a negative slope, second for azero slope, third for a negative slope, fourth for a zero slope, fifthfor a positive slope and sixth for a zero slope.

In FIG. 24 six boxes indicate a change of slope in the signal 120. A box121 indicates the start of a negative slope. Box 122 indicates the startof a zero slope. A box 123 indicates the start of a negative slope. Box124 indicates the start of a zero slope. A box 125 indicates the startof a positive slope. Box 126 indicates the start of a zero slope.

Upon finding the last zero slope, the computer 21 would know that it hasreached the end point of the process. Any of the detected changes inslope could signal the computer 21 to intervene in some way in theprocess. Additionally, instead of intervening or marking the signal 120at first detection of a change in slope, the computer 21 could wait one,two, three or etc. boxes after a change in slope is detected or couldwait an additional percentage of the real time from the beginning of theplasma process on to each end point before intervening in the process.

I claim:
 1. A method for displaying a signal which represents thevariation of light intensity over time in a processing chambercomprising the steps of:(a) detecting the light intensity within theprocess chamber to produce a voltage signal with a voltage amplitudevarying with the light intensity within the process chamber; (b)digitally sampling the voltage amplitude of the voltage signal at asampling interval (I) to obtain sample values (S_(x)); (c) displaying ina graph sequential values of the voltage signal along a y-axis versustime along an x-axis; (d) superimposing over the graph a rectangular boxhaving a height H representing an amount of light intensity extending inthe direction of the y-axis and a width W representing an amount of timeextending in the direction of the x-axis that includes earlier displayedvoltage signals and placed so that the midpoint of the left side of therectangular box is coincident with an earlier displayed voltage signal;(e) waiting for one of a positive slope, a negative slope and a zeroslope; (f) detecting whether a newly displayed voltage signal exits tothe right of the rectangular box, above the rectangular box or below therectangular box; (g) when the newly displayed discrete value of thevoltage signal is at the right of the rectangular box, designating theslope of the graph at the discrete value as having a zero slope; (h)when the newly displayed discrete value of the voltage signal is abovethe rectangular box, designating the slope of the graph at the discretevalue as having a positive slope; (i) when the newly displayed discretevalue of the voltage signal is below the rectangular box, designatingthe slope of the graph at the discrete value as having a negative slope;and (j) highlighting the rectangular box when the slope designated insteps (g), (h) and (i) is the waited for slope of step (e).
 2. A methodaccording to claim 1 wherein said processing chamber contains a plasma.3. A method according to claim 1 wherein step (b) includes the substepsof(b) converting the voltage signal to a frequency signal having afrequency which varies in accordance with the voltage amplitude of thevoltage signal; (b2) storing a running total of pulses of the frequencysignal in a counter; and (b3) sampling and clearing the counter toobtain the sample values.
 4. An apparatus for displaying a signal whichrepresents the variation of light intensity in a processing chamber overtime comprising:(a) detecting means, coupled to the processing chamber,for detecting the light intensity within the process chamber to producea voltage signal with a voltage amplitude varying with the lightintensity within the processing chamber; (b) sampling means coupled tothe detecting means for digitally sampling the voltage amplitude of thevoltage signal at a sampling interval (I) to obtain samples values(S_(x)); (c) display means for displaying in a graph sequential valuesof the voltage signal along a y-axis versus time along an x-axis and forhighlighting a change in the slope of the graph; and (d) means forchanging the processing conditions within said chamber in response tostep (c).
 5. An apparatus according to claim 4 wherein said processingchamber contains a plasma.
 6. An apparatus according to claim 4 whereinthe sampling means includes(b1) converting means for converting thevoltage signal to a frequency signal having a frequency which varies inaccordance with the voltage amplitude of the voltage signal; (b2)counter means, coupled to the converting means, for storing a runningtotal of pulses of the frequency signal in a counter; and (b3) computermeans, coupled to the counter means, for sampling and clearing thecounter to obtain the sample values.
 7. A method for monitoring aprocess within a reaction chamber comprising(a) detecting lightintensity of a plasma in the reaction chamber to produce a voltagesignal with a voltage amplitude varying with the light intensity; (b)displaying in a graph sequential values of the voltage signal along ay-axis versus time along an x-axis; (c) superimposing over the graph arectangular box having a height H representing an amount of lightintensity extending in the direction of the y-axis, and a width Wrepresenting an amount of time extending in the direction of the x-axisthat includes earlier displayed voltage signals and placed so that themidpoint of the left side of the rectangular box is coincident with anearlier displayed voltage signal; (d) programming a computer to monitorand display changes in the slope of the graph; and (e) intervening insaid process when an anticipated sequence of one or more slopes has beendisplayed.
 8. A method according to claim 7 wherein a box is highlightedwhen the change in slope of step (e) is detected.
 9. A method accordingto claim 7 wherein said box is advanced to include the latest voltagesignals when the number of voltage signals exceeds the width of the box.10. A method for displaying a filtered signal which represents thevariation of light intensity over time in a processing chamber wherein amagnetic field generated by a signal with a period T causes plasma torotate within the processing chamber, comprising the steps of:(a)detecting the light intensity within the process chamber to produce avoltage signal with a voltage amplitude varying with the light intensitywithin the process chamber; (b) digitally sampling the voltage amplitudeof the voltage signal at a sampling interval (I) to obtain sample values(S_(x)); (c) calculating running averages (R_(y)) of a preselectednumber (N) of sample values (S_(y)); (d) setting the period T equal tothe preselected number (N) times the sampling interval (I) times aninteger; (e) displaying in a graph sequential values of the voltagesignal along a y-axis versus time along an x-axis.
 11. A methodaccording to claim 10 wherein changes in slope of the graph are employedto make processing decisions.